Time Piece: Shock Value

FRAMINGHAM (08/15/2000) - There are two ways to determine whether a high voltage is present in a circuit. One is to spit on your thumb and index finger, place them across the circuit and wait for the jolt.

The other is to hook up a voltmeter.

The voltmeter is a classic instrument, still indispensable despite its age (the magnet-and-coil mechanism on which it is based dates back to the galvanometer, introduced by the French physician and physicist Jacques-Arsne d'Arsonval in the 1880s). Its range is astonishing--it can measure the thousands of volts on the deadly third rail of a train track as easily as it can check the voltage of a microcircuit found in a cell phone. Technicians might use it in combination with an ammeter, which measures current in amperes, and an ohmmeter, which measures resistance in ohms. These three instruments often can be found together in a so-called multimeter. This living arrangement makes a good deal of sense given the tight relationship among the variables in question. As you no doubt remember from your physics class, voltage equals the current times the resistance.

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